Re: [PATCH v8 0/7] vhost-user: Add SHMEM_MAP/UNMAP requests

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Sep 26, 2025 at 10:28:07AM +0200, Albert Esteve wrote:
> On Wed, Sep 10, 2025 at 1:54 PM Albert Esteve <aesteve@redhat.com> wrote:
> >
> > Hi all,
> >
> > v7->v8
> > - Unified VhostUserShmemObject and VirtioSharedMemoryMapping
> > - Refined shmem_obj lifecycle by transferring ownership
> > - Other small improvements
> > v6->v7
> > - Fixed vhost_user_shmem_object_new to use
> >   memory_region_init_ram_from_fd as before
> > v5->v6
> > - Added intermediate QOM object to manage shared MemoryRegion lifecycle
> >   with reference counting, and automatic cleanup
> > - Resolved BAR conflict, change from 2 to 3 to avoid conflict with
> >   `modern-pio-notify=on`
> > - Added SHMEM_CONFIG validation in vhost-user-test
> > - Changed VirtSharedMemory -> VirtioSharedMemory
> > - Changed MappedMemoryRegion -> VirtioSharedMemoryMapping
> > - Changed from heap-allocated MemoryRegion *mr to embedded MemoryRegion
> >   mr in VirtioSharedMemory structure to eliminate memory leaks and
> >   simplify cleanup
> > - Fixed VirtioSharedMemory initialization and cleanup with
> >   memory_region_init() and object_unparent()
> > - Other minor fixes, typos, and updates.
> >
> > This patch series implements dynamic fd-backed memory mapping support
> > for vhost-user backends, enabling backends to dynamically request memory
> > mappings and unmappings during runtime through the new
> > VHOST_USER_BACKEND_SHMEM_MAP/UNMAP protocol messages.
> >
> > This feature benefits various VIRTIO devices that require dynamic shared
> > memory management, including virtiofs (for DAX mappings), virtio-gpu
> > (for resource sharing), and the recently standardized virtio-media.
> >
> > The implementation introduces a QOM-based architecture for managing
> > shared memory lifecycle:
> >
> > - VirtioSharedMemoryMapping: an intermediate object that manages
> >   individual memory mappings by acting as generic container for regions
> >   declared in any vhost-user device type
> > - Dynamic Mapping: backends can request mappings via SHMEM_MAP messages,
> >   with the frontend creating MemoryRegions from the provided file
> >   descriptors and adding them as subregions
> >
> > When a SHMEM_MAP request is received, the frontend:
> > 1. Creates VirtioSharedMemoryMapping to manage the mapping lifecycle
> > 2. Maps the provided fd with memory_region_init_ram_from_fd()
> > 3. Creates a MemoryRegion backed by the mapped memory
> > 4. Adds it as a subregion of the appropiate VIRTIO Shared Memory Region
> >
> > The QOM reference counting ensures automatic cleanup when mappings are
> > removed or the device is destroyed.
> >
> > This patch also includes:
> > - VHOST_USER_GET_SHMEM_CONFIG: a new frontend request allowing generic
> >   vhost-user devices to query shared memory configuration from backends
> >   at device initialization, enabling the generic vhost-user-device
> >   frontend to work with any backend regardless of specific shared memory
> >   requirements.
> 
> Small bump to avoid this patch series falling too much.

Sorry for the slow review. I've finished now.

Stefan

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